A future wireless communication system requires a higher information transmission rate and communication quality. In order to realize the aim with limited frequency resources, MIMO (Multi Input Multi Output) technology has become one of the necessary technical means in the future wireless communication.
In the MIMO system, a signal is transmitted through multiple antennas at transmit-side and the signal is received through multiple antennas at the receive-side. Researches show, compared with traditional single antenna transmission methods, MIMO technology may improve channel capacity significantly and thus the information transmission rate.
Transmission method adopting pre-coding in the MIMO system may effectively improve a performance of the MIMO system. A basic idea of the pre-coding is to pre-process transmission data according to current channel information before the transmission data is transmitted, including linear processing and non-linear processing, etc.
The MIMO pre-coding is a kind of closed loop transmission technology and can be divided into two types according to required forms of feedback information, which are codebook based pre-coding and non-codebook based pre-coding.
In the codebook based MIMO pre-coding, a codebook is pre-set at the transmit-side and receive-side before communication and the codebook includes several pre-coding codewords. Before the transmission of each resource block (RB), the receive-side will select a best pre-coding codeword from the codebook according to current channel characteristics (at this time, if the transmit-side performs pre-coding with this codeword and then transmits the signal result, the receive-side achieves the best performance), and feed back an index corresponding to the codeword in the codebook to the transmit-side. The transmit-side selects the codeword corresponding to the index from the codebook to perform the pre-coding for the transmission data according the feedback information. Therefore, in the codebook based pre-coding, the form of the feedback information is the index corresponding to the codeword.
Compared with the non-codebook based pre-coding, the codebook based pre-coding may effectively reduce the feedback overhead of the channel and is a more practical pre-coding technology.
In a conventional solution, the codebook based pre-coding technology uses a single fixed codebook for all resource blocks in a time and frequency space.
For the multi-user scheduling that considers fairness to users, the larger a fluctuation of the channel characteristic of the user is, the bigger a gain of the scheduling (multi-user diversity gain) is. However, for a case that the fluctuation of a channel, such as a Rician channel, slow fading channel or related channel, is not large, when a single fixed codebook is adopted, since the size of the codebook is limited (limited by the feedback overhead), the fluctuation of the equivalent channel after the pre-coding with the codeword in the codebook is still inadequate, which leads to losses of the multi-user scheduling performance.